1. Field of the Invention
This invention relates to a soybean milk for long life tofu, and to a process for producing a long life tofu by the use of said soybean milk.
2. Description of the Prior Art
Reflecting the current tendency searching for a more improved health, tofu is being watched with a world-wide interest today.
From the viewpoint of commercial circulation, however, tofu has a difficult problem that it is susceptible to microbial pollution and therefore poor in preservability. As methods for solving this problem, long life tofu (hereinafter referred to as "LL tofu") which can be circulated at ordinary temperature, namely retort packed tofu, and aseptic packed tofu have been thought out, and various production processes thereof have been proposed (e.g. U.S. Pat. No. 4,000,326).
Since retort sterilization is a heat treatment under a high temperature-high pressure condition, it is unavoidably accompanied with some undesirable changes due to heating, such as formation of disagreeable odor, browning in color, change in taste, etc. Particularly since tofu is white in color and soft in taste and flavor, these changes exercise an important influence. Thus, retort packed tofu is not easy to produce.
On the other hand, in case of aseptic packed tofu, too, the soybean milk is exposed to high temperature (ca. 130.degree.-150.degree. C.) in the process of sterilization, followed by coagulation step at about 90.degree.-95.degree. C. by adding a coagulant to form an aseptic packed tofu. Thus, formation of disagreeable odor and browning in color unavoidably take place due to heating at a high temperature, similarly to the case of retort sterilization.
Further, LL tofu is stored for a long period of time at ordinary temperature and therefore its browning progresses even in the course of storage. Particularly when container has no oxygen barrier property, this tendency is remarkable and causes deterioration in quality.
The formation of disagreeable odor and browning in color due to heating of tofu are attributable to the soluble saccharides and free amino acids in soybean milk which react upon heating to form brownish materials. Accordingly, these changes due to heating will be preventable by removing these substances from soybean milk. Based on this thought, the present inventors previously established a process for producing LL tofu by removing soluble saccharides from raw soybean and producing LL tofu from a soybean milk prepared from thus treated soybean (U.S. Pat. No. 4,636,398; Japanese Patent Application Kokai (Laid-Open) Nos.60-149354, 61-289851, etc.).
In the above-mentioned process, soluble saccharides are removed by soaking raw soybean in hot water. Thereafter, based on a consideration that removal of soluble saccharides will be similarly achievable also by other methods, the present inventors reached an idea of ultrafiltration.
Ultrafiltration of soybean milk is disclosed in Japanese Patent Application Kokai (Laid-Open) Nos. 49-93557, 52-7466, 59-189986, etc. However, object of these methods consists in removing the disagreeable odor component or concentrating a soybean milk, which differs from the object of the present invention, namely preparation of a soybean milk suitable for producing retort packed tofu by removing soluble saccharides therefrom.
On the other hand, with diversification of dietary life, it is also requested to supply a tofu more firm than ever. As a method for it, addition of isolated soybean protein to soybean milk was proposed (Japanese Patent Application Kokai (Laid-Open) Nos. 58-78559, 62-195262, etc.).
However, addition of isolated soybean protein is disadvantageous in that the protein is difficult to therefrom has a salty taste due to the salt originated from the isolated soybean protein. In addition, this method requires to use an isolated soybean protein having a high gelling ability to endure the condition of sterilization. Apart from the above, in order to increase the protein concentration of soybean milk, it has also been proposed to reduce the quantity of water to be added to starting soybean or to concentrate soybean milk, as methods for preparing firm tofu. However, the former method is low in efficiency because of low recovery rate of protein.
As one example of the latter method, the use of selective permeable membrane for concentrating soybean milk is known. In this method, a reverse osmosis membrane or an ultrafilter membrane giving a relatively low fractional molecular weight (5,000 or below) is used in order to minimize the change in component, so that this is unsatisfactory as a method for producing firm LL tofu.